If Plex is using the UWP APIs supported on the XBox One then 3D playback is quite possible. The Blu-Ray app supports it, and I believe I may have found how.
Within the MediaPlayer object there is the settable property “StereoscopicVideoRenderMode”, and the method CopyFrameToStereoscopicVideoSurfaces which should enable you to implement 3D playback within the Plex app.
